The adrenal function mainly cortical one, was evaluated in 16 patients with incidentally discovered adrenal masses. Pathological examination was possible in 15 cases. The finding consisted of adrenocortical adenoma in 9, adrenocortical nodular hyperplasia in 1, adrenal medullary hyperplasia in 1, metastatic tumor in 2 and adrenal cyst in 2. Another case of adrenal cyst was diagnosed by percutaneous puncture. In all cases peripheral levels of plasma cortisol, plasma aldosterone concentration and plasma renin activity were normal. Plasma catecholamine levels were also normal except in a case of adrenal medullary hyperplasia. On the other hand, the cases of adrenocortical adenoma displayed elevation of urinary 17-hydroxycorticosteroids in 6/9 (67%), a loss of plasma cortisol circadian rhythm in 3/7 (43%) and insufficient suppression on dexamethasone (DXM) suppression test in 6/9 (67%). Their adrenal scintigraphy (with 131I-6 beta-iodomethyl-9-nor-cholest-5 (10)-en-3 beta-ol) revealed an increased ipsilateral up-take and insufficient suppression after DXM in all, while a diminished contralateral up-take in 4/9 (44%). These data suggested that a considerable number of adrenal incidentalomas may not be truly "non-functioning". Two patients with cortical adenoma experienced post operative adrenal insufficiency (25%). It was suggested that a pre-operative loss of plasma cortisol circadian rhythm was the most prognosticating of the post operative adrenal insufficiency, rather than insufficient DXM suppression or scintigraphic absence of contralateral up-take. Among the patients with malignancy, differentiation of incidental adrenal adenoma from metastasis by size alone may not be reliable.